A Superconducting Quantum Interference Device (SQUID) is a very sensitive magnetometer used to measure extremely small magnetic fields. SQUIDs function using two Josephson junctions connected in parallel in a superconducting loop. As used herein, SQUID refers to a DC-SQUID, and not to an RF-SQUID which uses a single Josephson junction.
An initial bias current, IB, is introduced and splits evenly between both branches of the loop, which encloses a certain magnetic flux, φ. FIG. 1 shows a SQUID 10 in its unperturbed state, when there is no external magnetic field, I1=I2=0.5*IB. An externally imposed magnetic flux, the quantity SQUIDs are used to measure, can change the value of the enclosed flux and consequently induce a current in the loop. The induced current flows around the loop and adds to the bias current in one branch but subtracts from it in the other branch. When the induced current exceeds a critical value, a voltage, V, appears across the SQUID.
A typical plot of the voltage across a SQUID responding to changes in the enclosed magnetic flux is shown in FIG. 2. The measured voltage will vary sinusoidally with the magnetic flux with a period proportional to the magnetic flux quantum, φ0. Of note is the fact that any particular voltage measured across the SQUID may correspond to any one of a theoretically infinite number of possible values of the magnetic flux.
FIG. 3 shows SQUID 10 configured as a measurement instrument. In this configuration, the external magnetic field is imposed by an input current, Iin, passing through an inductor, L1, near SQUID 10. SQUID controller 20 supplies the bias current, IB, and measures the voltage across SQUID 10. As the input current changes, the magnetic flux through SQUID 10 changes and the voltage measured by SQUID controller 20 changes in the manner illustrated in FIG. 2. There are a potentially infinite number of possible values of the magnetic flux for any one measured value of the voltage across SQUID 10.
To achieve an approximately linear measurement of the magnetic flux passing through the loop a re-balancing control system is used where a feedback controller measures the voltage across the SQUID and adjusts the feedback current flowing through feedback inductor L1F in order to counteract the changes in flux imposed on SQUID 10 by input inductor L1 and keep the measured voltage constant. The value of the voltage to be maintained is chosen to be the average value of the sinusoid so that small variations are approximately linear with respect to magnetic flux.
However, SQUID controller 20 is limited in its ability to detect a voltage change and adjust the feedback current to compensate. High frequency or high amplitude changes in the input current, causing a high slew rate in the measured voltage across SQUID 10, can overwhelm the ability of SQUID controller 20 to adjust the compensating feedback current quickly enough. This results in SQUID controller 20 “unlocking” and settling into a different value of the magnetic flux for the same measured voltage than before the unlocking event took place.